Workers battling to prevent nuclear meltdown at the Fukushima power plant will use a police water cannon to try to cool one of the crippled reactors after strong winds and high radiation levels thwarted attempts to spray from a helicopter.

The renewed efforts came as the EU's energy chief, Guenther Oettinger, said the plant was "effectively out of control" and the head of Russia's state nuclear corporation said the situation in Japan was "developing under the worst scenario".

Yukiya Amano, the head of the International Atomic Energy Agency – who has urged the Japanese government to provide better information about the nuclear crisis – also plans to go to Japan.

Technicians were evacuated from the most dangerous areas of the plant on Wednesday morning after radiation levels became too hazardous for them to remain.

Their withdrawal – and the failure of a plan to drop water by helicopter – have further hampered efforts to secure safety at the atomic power plant and avert a major radiation leak.

NHK TV said police would now attempt to cool the No 4 reactor using water from a cannon truck, while Japan's nuclear safety agency said the military would also help pump water into that reactor's spent fuel pool as well as No 3 reactor. However, another attempted helicopter mission is likely, according to the agency.

Workers at the plant cleared debris to build a road so fire trucks could reach reactor No 4 at the complex 150 miles north of Tokyo. The plant operator described No 3 – the only reactor at that uses plutonium in its fuel mix – as the priority.

The situation at No 4 reactor, where the fire broke out, was "not so good", the plant operator added. Water was also being poured into reactors No 5 and 6, indicating that the entire six-reactor facility was now at risk of overheating.

"Getting water into the pools of the No 3 and No 4 reactors is a high priority," Hidehiko Nishiyama, a senior official at Japan's Nuclear and Industrial Safety Administration, told a news conference, adding that the pool for spent fuel rods at No 3 was heating up while No 4 remained a concern.

"It could become a serious problem in a few days," he said.

Emperor Akihito said he was "deeply concerned" about the nuclear crisis and was praying for his people.

In his first public comments since last Friday's massive earthquake, Akihito appeared live on Japanese television to offer his condolences to the victims of the disaster.

"I am deeply concerned about the nuclear situation because it is unpredictable," he said. "With the help of those involved I hope things will not get worse."

The nuclear plant's operator, Tokyo Electric Power Company (Tepco), had earlier warned that plans to spray the reactor with water and boric acid in an attempt to slow down the nuclear reaction and cool the core might not work.

"It's not so simple that everything will be resolved by pouring in water. We are trying to avoid creating other problems," he said.

"We are actually supplying water from the ground, but supplying water from above involves pumping lots of water, and that involves risk. We also have to consider the safety of the helicopters above," he said.

The 70 or so engineers, working around the clock in harsh conditions, spent Wednesday morning trying to put out a fire at one reactor and to cool others.

To compound problems, a fire broke out at the No 3 reactor, where a fuel storage pool had overheated and may have let off radioactive steam. Live TV footage showed a large cloud of light grey smoke rising above the plant.

All six reactors at Fukushima are experiencing problems following the earthquake and tsunami, in which an estimated 10,000 people died.

The technicians were ordered to leave the facility after the level of radiation at the plant soared to 10 millisieverts per hour – above the level considered harmful to human health – possibly as a result of radioactive substances being emitted from the No 2 reactor. The reading later fell to around six mSv per hour, reports said.

The evacuation followed another day of crisis at the plant, which has become the focus of the world's attention, even as rescue workers sift through the damage caused by the tsunami along a vast stretch of Japan's north-east coast.

Earlier, officials from the nuclear and industrial safety agency said that 70% of fuel rods at the No 1 reactor had been significantly damaged, as well as 33% of rods at the No 2 reactor. The cores of both reactors are believed to have partially melted, Kyodo news agency said.

"We don't know the nature of the damage," said Minoru Ohgoda, spokesman for the country's nuclear safety agency. "It could be either melting, or there might be some holes in them."

Before they were moved to safety the workers had been trying to cool spent nuclear fuel pools at the No 5 and No 6 reactors, where temperatures have risen above normal levels.

The government has ordered 140,000 people living within a 19-mile radius of the plant to remain indoors after a spike in radiation levels. A further 70,000 residents had already been moved to safe distances. The government said it had no immediate plans to widen the evacuation zone.

The crisis unfolding in Fukushima continued to raise anxiety levels in Tokyo, 150 miles to the south. Radiation levels in the capital were 10 times higher than normal on Tuesday evening, but posed no health hazard, the government said.

The meteorological agency said winds near the power plant would blow from the north-west and out into the Pacific Ocean on Wednesday. The winds were expected to strengthen in the afternoon, the agency added.

US forces in Japan are not allowed within 80 km (50 miles) of the crippled plant, according to the Pentagon.

The US military is also giving some flight crews potassium iodide tablets before of missions as a way to guard against effects of radiation, officials said.

The measures were seen as precautions and the Pentagon said no US forces have shown signs of radiation poisoning.